1. Field of the Invention
The present invention relates to the field of use of digital image capturing. More particularly, the present invention relates to image sensors with built-in analog-to-digital converters.
2. Description of Related Art
Typically, imaging sensor arrays have separate analog-to-digital (A/D) circuits which are located off of the array to digitize the captured pixels. The array is composed of a grid of pixel sensors, each pixel sensor generating a charge when the pixel sensor is exposed to a light source to which it is directed.
During operation, the array first captures the image by having each pixel sensor generating and storing an amount of charge corresponding to the amount of light it is exposed to. Then, the array sends the charge stored in each pixel sensor to the A/D circuit over an analog transfer circuit. The A/D circuit then produces a digital representation of the amount of charge received from the pixel sensor.
As the A/D circuit is often located some distance away from the array, the length of the analog lines which are used to send the analog pixel data between the array and the A/D circuit are quite long. As the length of the analog lines is increased, noise and power effects must be taken into consideration to avoid the loss or degradation of the captured signals.
Due to noise considerations, the number of analog lines used to transfer pixel signals from the array to the A/D circuit may be limited to avoid cross-talk or any of the other problems which are caused by interference between these signal lines. Additionally, if multiple independent A/D circuits are used, mismatches between them can cause undesirable artifacts in the resulting image. Therefore, the pixels in an array are typically digitized by transferring each pixel's charge sequentially to a single A/D circuit. In color sensors, where the array may be considered to consist of several sub-arrays each corresponding to a particular color channel, each color channel may have a single A/D circuit.
It would be desirable to eliminate the difficulties involved in accurately transferring the accumulated charge in each pixel to the A/D circuit. It may also sometimes be desirable to perform some amount of highly-parallel digital processing on the captured image before outputting it from the sensor--also known as digital focal-plane processing.
One method of accomplishing accurate transfers and parallel pixel on-sensor processing would be to perform the analog to digital conversion within the circuitry of each pixel However, to accomplish A/D conversion within the circuit of each pixel would require each pixel to have its own A/D circuit and, thus requiring the building of a conventional A/D circuit and increasing the cost of each pixel.
Thus, it would be desirable to be able to accomplish A/D conversion within a pixel without requiring each pixel to have the extra circuitry and cost associated with conventional A/D circuits.